The present invention relates to coating methods. More particularly, the invention relates to an improved coating method for forming a coated magnetic recording medium.
The term "coated magnetic recording medium" as herein used is intended to mean a magnetic recording medium which is manufactured by a method in which, while a non-magnetic, flexible and belt-shaped suport (hereinafter referred to as "a web" when applicable) is being transported along a predetermined path, a suitable coating liquid is applied to the surface of the web to a desired thickness. This manufacturing process is greatly different from a so-called "non-coated magnetic recording medium" manufacturing process which utilizes ionplating, sputtering or vacuum deposition.
Coated magnetic recording mediums have been manufactured for recording audio or video signals or for storing digital data using any of gravure, reverse roll, doctor blade or extrusion coating methods. It is difficult with each of the aforementioned conventional methods to greatly increase the coating speed as would provide an increased production rate.
The main factors making it difficult to increase the coating speed are that the thickness of the support is very small, and of the various coating solutions, especially a coating solution for forming a ferromagnetic layer has a thixotropy effect, or so-called "thixotropic properties", which greatly affects the electromagnetic conversion characteristic.
If, in a coating system having a relatively intricate web path, a thin web is run at high speed, the web has a tendency to be abnormally shifted to one side, bent, creased, or broken.
In a coating system in which the path of a coating solution for forming a ferromagnetic layer has relatively many points where the coating solution can accumulate, the viscosity of the coating solution is liable to increase to an abnormal value or the coating solution is liable to aggregate because of the aforementioned thixotropy property. This may change the electromagnetic conversion characteristic of the ferromagnetic layer greatly.
U.S. patent application Ser. No. 171,509, filed July 23, 1980 by the present applicant disclosed an improved coating method which is intended to eliminate these difficulties. In accordance with that method, an extrusion technique and a doctor blade technique are combined. In the coating method, as shown in FIGS. 1 through 3, a web 4 run continuously at high speed is coated with a magnetic layer coating solution 3 when passing a backing roll 7. The coating solution 3, after being supplied into the solution pool 10 of an extruder 1, is discharged at a predetermined velocity through a slit 2 above the solution pool 10. Excessive amounts of coating solution 3 are scraped off from the coated web 4 by a doctor blade 9 as the web 4 passes a doctor backing roll 8. The doctor blade 9 is disposed to confront the web adjacent the backing roll 8. A smoother 11a is provided between the backing roll 7 and the doctor backing roll 8 in order to smooth the coated film. Similarly, a smoother 11b is disposed downstream of the backing roll 8 to smooth the coated film metered by the doctor blade.
The solution pool 10 is in the form of a cylinder having a diameter of 15 to 20 mm which extends throughout the extruder 1, communicates with the bottom of the slit 2 and is substantially parallel to the slit 2, as shown in FIGS. 2 and 3. One end of the cylindrical solution pool 10 is connected to a solution supplying nozzle 12 while the other end is closed by a closing plate 13 so that, after being filled in the solution pool 10 through the nozzle 12, the coating solution 3 is discharged in the form of a layer through the slit 2. That is, the coating solution 3 is supplied at a predetermined flow rate through the nozzle 12 and is completely discharged through the slit 2. Coating at a high speed of more than 120 m/min can be achieved by the employment of the coating method described above.
However, it has been found that the method still leaves problems to be solved specifically, problems related to the effect of thixotropy of the coating solution on the formation of a ferromagnetic layer and the flow rate of the coating solution supplied to the solution pool 10. For instance, if the amount of coating solution scraped off by the doctor blade 9 is reduced to suppress the occurrence of aggregation due to the recovery of the coating solution scraped off, or the flow rate of the coating solution supplied to the solution pool 10 is decreased so that the peripheral units can be miniaturized with the doctor blade 9 removed, then the coating solution 3 discharged through the portion of the slit 2 which is closer to the solution supplying nozzle 12 is liable to cause non-uniformity in color in the widthwise direction of the web 4, while the coating solution 3 discharged through the portion of the slit 2 which is closer to the closing plate 13 is liable to have stripes formed therein distributed in the widthwise direction of the web 4. It has been confirmed that the above-described color non-uniformity and stripes adversely effect the electromagnetic conversion characteristic of the ferromagnetic layer and result in noise on the tape.
The creation of the stripes may be eliminated by increasing the flow rate of the coating solution. The creation of the stripes by the coating solution 3 which is discharged through the portion of the slit 2 closer to the closing plate 13 is thereby eliminated. However, it has been found that the region where the color non-uniformity occurs tends then not only to include the vicinity of the solution supplying nozzle 12 but also to spread towards the closing plate 13.
The inventors have conducted intensive research and experimentation to determine the causes for the occurrence of the above-described color non-uniformity and stripes and found that the flow rate of the coating solution in the longitudinal direction in the solution pool 10 significantly governs the thixotropy properties. More specifically, the viscosity of the coating solution 3 in the vicinity of the solution supplying nozzle 12 where the flow rate is relatively high tends to be lowered because the coating solution is subjected to shearing action which causes the color nonuniformity. Also, the coating solution in the vicinity of the closing plate 13 where the flow rate is decreased substantially to zero tends to aggregate thus creating the stripes.
An object of the invention is thus to provide a coating method and apparatus in which the above-described difficulties accompanying a conventional coating method have been eliminated, but yet in which the solution supplying means, the coating means, the metering means and the smoothing means are simple in structure.